Tested servers:
- Xandikos: Everything works ✅
- Radicale: Everything works ✅
- Cyrus IMAP: One issue reported. Also, support not advertised.
- Nextcloud: One issue reported. Also, collections have no Etag.
- Baikal: Collections have no Etag.
- Icloud: Missing test account
Need to figure out how to automate a report for this.